1. Field of the Invention
The present invention relates to a wavelength tunable surface emitting laser and an optical coherence tomography apparatus including the surface emitting laser.
2. Description of the Related Art
Wavelength tunable lasers, which are capable of changing a laser oscillation wavelength thereof, are expected to be applicable to various fields, such as communications, sensing, imaging, etc., and therefore have been widely researched and developed in recent years.
A wavelength tunable vertical-cavity surface emitting laser (VCSEL) structure is an example of a wavelength tunable laser. The wavelength tunable VCSEL structure controls a laser oscillation wavelength of a vertical-cavity surface emitting laser (VCSEL) by using micro electro mechanical system (MEMS) technologies.
VCSELs generally include an active layer that is sandwiched between a pair of reflectors, such as distributed Bragg reflectors (DBRs), and a laser beam is oscillated at a wavelength corresponding to a cavity length determined by an optical distance between the reflectors. In wavelength tunable VCSELs according to the related art, the cavity length is changed by moving one of the reflectors while an air gap is provided between one of the reflectors and another layer (generally a semiconductor layer), so that the laser oscillation wavelength is changed accordingly (see U.S. Pat. No. 6,549,687).
An amount of change in a laser oscillation wavelength of a wavelength tunable VCSEL relative to an amount of movement of a reflector is sometimes referred to as a wavelength tuning efficiency. As the wavelength tuning efficiency increases, an amount of movement of the reflector required to cause a certain wavelength change decreases. Accordingly, a voltage required to mechanically drive the reflector decreases. To increase a wavelength tuning speed, the resonance frequency of the reflector may be increased. In such a structure, the spring constant is generally increased, and the amount of movement of the reflector is reduced. When the wavelength tuning efficiency is high, since the amount of movement of the reflector required to cause a certain wavelength change is small, the wavelength tunable range and the wavelength tuning speed can be increased at the same time.